The present invention is directed generally to apparatus for and improvements in powdered metal consolidation processes, some of which referred to as "electroconsolidation" processes, and more particularly to processes employing specially shaped rams or electrodes and/or other press elements for controlling the pressure and/or electrical flux (current density) within a die chamber to control the rate and uniformity of heating of a workpiece to be consolidated.
Electroconsolidation is one of a class of "pseudo-fluid" or "pseudo-isostatic" processes for consolidating preformed materials into highly densified components. In general, prior art "pseudo-fluid" or "pseudo-isostatic" processes have utilized a bed of solid particles as a pressure transmitting medium. The solid particles which typically comprise the medium for such prior art processes thus serve as "soft-tooling" to enable pressure to be applied to an article in shapes of various complexities that could not simply be pressed between the flat-faced punches of the usual "hot-press" apparatus. One purpose of such "pseudo-fluid" or "pseudo-isostatic" processes is to permit manufacture of near net-shaped components of high density from lower density preformed shapes, to permit production of finished goods with a minimum of post-pressing processing, and to accomplish this result more rapidly and at lower cost than the conventionally established hot isostatic pressing (HIP).
Numerous methods that utilize solid particles as pressure transmitting media have been described in the prior art. Lange and Terwilliger [Lange, F.F. and Terwilliger, G.R., "The Powder Vehicle Hot-Pressing Technique", Am. Cer. Soc. Bull., Vol. 52, No. 7 (July 1973)] discuss in detail the purposes and technical considerations for using a "secondary powder" as a "powder vehicle" for conveying pressure to a heated object (preform) to be densified. Ferguson and co-workers-described the "Ceracon process" which utilizes spherical-shaped grains of high elastic modulus as the medium for conveying the consolidation pressure to the preform. [Ferguson, B.; Kuhn, A.; Smith, O.D.; and Hofstatter, F., "Hot Consolidation of Porous Preforms Using Soft Tooling", Int. Journ. of Powder Met. & Powder Tech., Vol. 20, No. 2 (1984)]. A series of background patents regarding the present invention include those of U.S. Pat. No. 4,041,123 to Lange et. al., U.S. Pat. No. 4,539,175 to Lichti et. al., and U.S. Pat. No. 4,640,711 to Lichti et. al. (1987). The Lichti and Hofstatter '175 and '711 patents describe the use of a spherically-shaped product of graphitic or other carbonaceous material as a pressing medium for the consolidation process.
U. S. Pat. No. 4,853,178 to Oslin et. al., and U.S. Pat. No. 4,933,140 to Oslin et. al. pass electrical current through the grain to effect the heating of the grain in the prior art process of the Lichti et. al. '175 and '711 patents. Whereas the Oslin patents teach the electrical heating of the particles of the pressure transmitting medium prior to compacting of the preform (body) into the desired final dense body, the electroconsolidation process employs electrical (electrothermal) heating after the application of the compaction pressure; because of this, the design of the die and ram assemblies is more critical in the electroconsolidation process than in the earlier pseudo-isostatic processes. The design of the ram assemblies as utilizable in the type of electroconsolidation process hereof and other processes is one principal subject of this invention.
As noted, in the method of electroconsolidation, as set forth for example in co-pending application Ser. No. 544,046, filed on Dec. 14, 1990 to Goldberger, the pressure transmitting medium is electrically conductive, and electric current is made to flow through the compacted medium, thereby to cause electrothermal heating to raise the temperature of the preformed work piece to the desired temperature for effective consolidation. Thus, in the electroconsolidation process, the solid particles are used, not only to transmit compaction pressure, but also to serve as an electrical resistor for the conversion of electrical energy into heat.
Despite the greater utility of these prior art processes as compared to yet earlier methods and apparatus, certain improvements have been possible and desirable.
Wherefore, it is a material object of the present invention to provide such improvements and to materially alleviate certain problems associated with the prior art.